This invention relates, in general, to sensors and more particularly, to a sensor having an integrated high current drive capability.
A wide variety of solid state sensor devices are available which produce electrical output in response to environmental conditions. The sensors can be broken generally into several basic categories: mechanical sensors, temperature sensors, magnetic field sensors, optical sensors, and chemical sensors. Sensor devices are often identified by a particular application such as biomedical sensor and gas sensors, but these generally fit into one or more of the basic categories.
In general, sensors tend to be temperature sensitive. Also, most sensors are effected by electrical noise. These factors have made it difficult to integrate high current devices with sensors because high current devices operate at high temperatures and produce significant electrical noise. Where it has been necessary to control high current loads with a sensor device, multiple devices have been used to avoid these noise problems. Multiple device solutions are both more expensive and less reliable than single device solutions. It is desirable to have a single device which can perform both sensing and produce a high current output controlled by the sensed signal.
Recently, integrated sensors have become available with both passive and active compensation circuits built on the same chip as the sensor device. Compensation has been limited primarily to temperature compensation. Examples of such devices in pressure sensors are found in U.S. Pat. No. 4,463,274 and U.S. Pat. No. 4,465,075 issued to Craig C. Swartz. Such devices are useful in providing some preconditioning to the sensor output before the output signal is passed to the other circuitry. Some pressure sensors are commercially available with integrated signal amplification capability. These devices, however, are not able to deliver high current to a load.
Although high current devices, such as DMOS and bipolar transistors, and sensors are both solid state devices, their differences in construction and operation have led the semiconductor industry to shun their integration. Pressure sensors and accelerometers, for example, required delicate micro-machining and carefully controlled processing which has not been compatible with MOS devices. Micro-machining requires silicon etching which weakens silicon substrates and increases breakage during processing. High current transistors, on the other hand, are complex devices which are too expensive to risk breakage. In the case of non-mechanical sensors, high current devices can be adversely affected by exposure to the environmental signal such as light, a magnetic field, or chemical exposure. Until now, high current devices have not been designed to operate in these environments. The fact that high current device designers have little familiarity with sensor design and vice versa has also slowed progress and limited recognition of the need for sensors having high current drive capability, and the particular problems associated with their integration.
Accordingly, it is an object of the present invention to provide solid state sensor monolithically integrated with a high current device.
Another object of the present invention is to provide a means for isolating the high current device from an environmental signal to which the sensor is exposed.
A further object of the present invention is to provide a sensor using processes which are common to high current device manufacture.